Hi Luis,
I am not a professional however I am sure that solid state SD or CF cards work like hard disk drives - that is "on paper" there is an infinite number of times these cards can be formatted. But like all storage devices they can fail at any time. I would recommend transferring prior images to a PC after the shoot & formatting the card before a new shoot. In that way you have all your images stored on another device & this leaves you with maximum space available for new data and minimum data loss in the event of a card failure.

Hi Luis, all flash memory has a finite lifespan and a certain number of 'writes' they'll support before giving up! It is quite a few though, so you shouldn't need to worry for camera use, but it is a concern for anyone wanting to run a standard operating system on flash memory as the constant rewrites of virtual memory can send flash to an early grave! Interestingly Vista's Readyboost apparently doesn't write constantly, hence Microsoft's claims it won't harm a USB key.